Re: [PATCH v2] xen-pciback: Consider INTx disabled when MSI/MSI-X is enabled

[Jan Beulich <jbeulich@suse.com>]

On 18.11.2022 03:35, Marek Marczykowski-Górecki wrote:
> Linux enables MSI-X before disabling INTx, but keeps MSI-X masked until
> the table is filled. Then it disables INTx just before clearing MASKALL
> bit. Currently this approach is rejected by xen-pciback.
> According to the PCIe spec, device cannot use INTx when MSI/MSI-X is
> enabled.

Similarly the spec doesn't allow using MSI and MSI-X at the same time.
Before your change xen_pcibk_get_interrupt_type() is consistent for all
three forms of interrupt delivery; imo it also wants to be consistent
after your change. This effectively would mean setting only one bit at
a time (or using an enum right away), but then the question is what
order you do the checks in. IOW I think the change to the function is
wrong.

Furthermore it looks to me as if you're making msi_msix_flags_write()
inconsistent with command_write() - you'd now want to also permit
clearing "INTx disable" when MSI or MSI-X are enabled. Which, I think,
would simply mean allowing the domain unconditional control of the bit
(as long as allow_interrupt_control is set of course).

Especially with these further changes I'm afraid at least for now I
view this as moving in the wrong direction. My view might change in
particular if the description made more clear what was wrong with the
original change (476878e4b2be ["xen-pciback: optionally allow interrupt
enable flag writes"]), or perhaps the discussion having led to the form
which was committed in the end.

Jan